The History of Astronomy

Transcription

1 The History of Astronomy plato.lib.umn.edu/

2 The Model of the Cosmos It s a natural thing for people to want to understand the world around them. People build models, a (simplified?) conceptual framework that represents the real world and operates in a manner consistent with observations. To be successful, a model MUST be able to do two things

3 Model Building A model must accept and incorporate all careful, accurate observations. No observations may be conveniently discarded, simply because they contradict the model. [The model must be refined!] A model must be able to make accurate predictions of future events.

4 Models Change Take note of how the model of the cosmos changes as new observations become available. Take note also of the forces which resisted logical changes: mental inertia, political censorship and religious dogma.

5 The First Astronomers The first astronomers were the Babylonians. Other cultures observed the stars, but the Babylonians were the first to keep records of the positions of objects in the sky.

6 Babylonian Astronomy The Babylonians kept careful records on clay tablets for 1400 years, from 1600 B.C. until 200 B.C., even though they were repeatedly conquered, and their culture declined. The tablet above records a lunar eclipse.

7 Religious Motivation To the Babylonians, the objects moving regularly or erratically through the sky were deities, gods who could influence the events and futures of men. Marduk was their chief. Marduk

8 Religious Motivation Because the motivation for the Babylonian s astronomy was religious rather than scientific (they wouldn t even have recognized the word!), the Babylonians never bothered to make a model of what the cosmos (universe to them, solar system to us) was like.

9 The Ancient Greeks The Greek culture overlapped the end of the Babylonian period (600 to 100 B.C.) Certain Greek sub-cultures (notably the Athenians) came to prize logic and philosophy as the way to understand the world. You might recognize some of the names: Pythagoras, Plato, Aristotle, Democritus, Aristarchus, and Hipparchus.

10 Pythagoras The philosopher Pythagoras was the first to propose a model of the cosmos. Because of their symmetry, Pythagoras thought that some shapes were more perfect than others.

11 Pythagoras (2) Because the sphere was perfectly symmetrical in every direction, it was considered to be the most shape. The sky and everything in it appeared to be spherical. It was also thought that the earth, being the center of life and thought, was spherical and therefore perfect. It follows that things that are perfect don t ever need to change.

12 Pythagoras (3) Pythagoras proposed a series of nested, concentric, crystalline spheres which rotated at constant speeds around the earth. Each sphere would carry a single object on its inner surface. The moon was on the closest sphere, followed by Mercury, Venus, the Sun, the outer planets. The stars were painted on the outermost sphere. The spheres were each able to rotate independently at different, but uniform speeds.

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14 Plato Plato was an extremely influential person in ancient Greece. Because he was so highly regarded, what he said was often taken as absolute truth.

15 Plato (2) Plato didn t change Pythagoras model of the cosmos, but Plato s contribution to the model-building process was his insistence that any model save the appearances. By this, Plato meant that any model of the universe had to accurately match the positions of the objects in the sky.

16 Plato (3) Plato s belief in the Pythagorean model and his insistence that a model must save the appearances became driving forces in the early understanding of the arrangement of the cosmos. (Note, however, that Plato himself couldn t live up to his own pronouncements: Pythagoras model couldn t explain retrograde motion of the planets!)

17 Aristotle Aristotle was a student of Plato s. He was the first to try to understand not just the way the cosmos was arranged, but the why and how of its functioning.

18 Aristotle (2) A Greek philosopher, Democritus, had proposed that every different substance in the universe had its own type of atom. Imagine cheese, wood, hair atoms! A competing idea was that there were only 5 elements, fire, air, water, earth, and ether and that every different substance was composed of different ratios of these elements.

19 Aristotle (3) Aristotle believed that the elements would always try to return to their sources. No forces were necessary for this to occur; rather it was natural for these movements to happen.

20 Aristotle (4) Some examples: Smoke rises, because it is mostly air. Flames rise because they were trying to return to the sun. Water flows downhill because it s attempting to return to the sea. A thrown rock falls because it s returning to the earth.

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22 Natural Motion Aristotle used arguments like these to explain that Pythagoras crystalline spheres turned naturally; no force or engine was required to drive the revolution. Today, we reject natural motion and accept something equally non-intuitive: action at a distance, forces like gravity and magnetism, transmitted with no visible mechanism or connection.

23 Aristotle (5) Aristotle also had other reasons for believing that the earth didn t move, but that the heavens did: He reasoned that if the earth were moving, a stone thrown into the air would fall back to the earth along a parabolic track, not a vertical track.

24 Aristotle (6) An arrow fired directly north, would appear to veer to the west as the earth under the arrow rotated eastward. Since neither of these things appeared to happen, it was quite natural for Aristotle to believe that the earth did not rotate.

25 A competing idea Aristarchus of Samos proposed a heliocentric model of the cosmos. It incorporated a rotating earth which revolved around the sun, along with all the other planets.

26 A Prediction Why wasn t Aristarchus model accepted? It made the prediction of heliocentric stellar parallax, which wasn t observed. Parallax is the apparent movement of a nearby object that is really due to the movement of the observer.

27 Angle α > Angle β

28 A Modern Explanation of Parallax

29 Parallax (2) Today, with large telescopes, we are able to detect and measure the parallax of nearby stars. Even the nearest stars show parallaxes of less than about 0.3 arcsecond, an apparent motion far too small to see with the naked eye. Parallax is real, but the ancients couldn t see it without the technology of the telescope.

30 Problems begging for solutions After the time of Aristotle, there were several problems that the model of the cosmos couldn t explain. Retrograde motion of the planets. The apparent speeding up and slowing down of the sun and planets at different times of the year. Varying shapes and durations of the planetary retrograde motions.

31 Try these animations o101/java/parallax/parallax.html allax/parallax.htm